The present invention relates to a cellular radio communication system. In particular the present invention relates to a broadband wireless access system suitable for the delivery of multi-media services.
There is a growing demand for broadband wireless access systems which can deliver the high data rates required for the provision of multi-media services. Such wireless access systems operate within licensed frequency bands. Accordingly, these systems are continually developing to carry more data across the limited frequency band allocated to them. Pressure for this development is two fold. Firstly, there is increased demand for multi-media services from subscribers to the system. Secondly, revenue for the network operator will increase as billing is calculated on a per byte of information delivered basis as opposed to on a per second basis.
The performance of wireless access communication systems is prone to dynamic degradation, ie. time variant degradation, due to changing environmental conditions. Wireless transmissions in the frequency range from 10 to 50 GHz are particularly prone to dynamic degradation resulting from rain and from the growth and movement of foliage located in the transmission path. For example, radio wave propagation through rain causes absorption and scattering of the radio energy. These effects cause signal attenuation and must be considered in the design of radio communications systems, particularly at frequencies above several GHz, as the attenuation effects increase with frequency. FIG. 1 shows the attenuation of a 30 GHz signal in dB per kilometer due to rainfall against the percentage of time that such rainfall occurs within climate zone xe2x80x98Fxe2x80x99 which zone covers the UK.
This type of dynamic degradation has been taken account of in existing wireless access systems by designing the systems for operation in worst case environmental conditions. This has been achieved by the use of robust modulation schemes such as QPSK (Quadrature Phase Shift Keying), also known as 4-QAM (Quadrature Amplitude Modulation) which deliver low BERs (bit error rates) of the order of 10xe2x88x929, ie. one incorrect bit per 109 bits transmitted, in poor environmental conditions. However, designing such systems for worst case environmental conditions in this way results in low rates of data transmission.
Amplitude dependent modulation schemes, such as 64-QAM, 16-QAM and QPSK in which the amplitude of the modulation envelope is varied according to the symbol to be transmitted (referred to hereafter as amplitude dependent modulation schemes) are considered to be the most spectrally efficient modulation schemes. These modulation techniques have therefore been preferred in radio communication systems such as mobile, satellite and fixed networks where the available bandwidth is limited. When using amplitude dependent modulation schemes, linear amplification is required in order to keep third order intermodulation products and spectral regrowth to within acceptable limits. Thus, some of the potential gain available from amplification has to be sacrificed.
As can be seen from FIG. 1, for the majority of time transmission conditions are good. Adaptive modulation techniques have been proposed which enable higher data rates to be achieved by the use of 16-QAM or 64-QAM modulation schemes when the transmission conditions across a wireless link are improved or where the distance over which the link extends is relatively short. In this way the rate of data transmission within a limited frequency band can be improved. However, for very poor transmission conditions or for transmissions over longer distances a break in communication can occur. This is because for amplitude dependent modulation schemes, the modulated signal has to undergo substantially linear amplification before transmission, as discussed above. This limits the output power of the amplifier, thus limiting the power of the signal that can be transmitted over the transmission link.
In known cellular wireless access system a frequency plan is implemented over a geographical area. The frequency plan allocates channels within the frequency band to localised cells and due to attenuation of a radio signal across the cells, the same channel can be re-used within other cells in the frequency plan. The aim is to maximise frequency re-use without causing interference between parts of the frequency plan which use the same channels. Generally, a base station is associated with a cell to transmit radio frequency signals to all end user terminals or CPEs (Customer Premise Equipments) located within the geographical area covered by the cell. The uplink from the CPEs in the area to the base station may be a common medium access uplink, for example a FTDMA (Frequency or Time Division Multiple Access) uplink in which time and frequency carrier slots can in some way be allocated for use by the CPEs to send signals to the base station. The downlink from the base station to the CPEs may be a TDA (Time Division Access) downlink, with time slots over which the base station sends signals to the CPEs.
Another approach to optimising the use of bandwidth is automatic repeat request (ARQ). In this approach the receiving unit, be it a base station or a CPE, detects which signals sent across the transmission link have been received with errors in them and sends a feedback message to the transmitting unit requesting that the signals which have not been correctly received are sent again. This is an alternative way of increasing or decreasing the amount of information which is sent across the transmission link dependent on environmental conditions. In poor transmission conditions, more data will have to be re-sent and so data rates will be low. In good transmission conditions, less data will have to be re-sent and so data rates will be higher. However, this method has a degree of transmission delay inherent within it which may not be appropriate for all multimedia services. It is also inefficient in terms of the amount of uplink or downlink resource used for services requiring a low bit error rate.
It is also known to use forward error correction (FEC) in which a FEC code is added to the data payload of a packet sent across a transmission link. The FEC code is used by the receiving unit to detect and correct errors in the data payload received by the receiving unit.
The present invention seeks to provide an improved cellular radio communication system which uses adaptive modulation and which can provide transmission links having optimised data rates with low bit error rates dependent on the prevailing transmission conditions, while maintaining the links in the poorest transmission conditions. Because links can be maintained in poor transmission conditions the present invention can enable operation over longer distance transmission links.
According to a first aspect of the present invention there is provided a cellular radio communication system for transmitting data over a plurality of transmission links comprising means for generating a modulated signal by applying a constant amplitude envelope modulation scheme to data to be transmitted across poor quality transmission links and amplifier means for non-linearly amplifying the modulated signal. By using a constant amplitude envelope modulation scheme higher gain non-linear amplification can be used without prejudicing the ability to recover data from the modulated signal. Thus, signals transmitted over the poorest quality transmission links can have a higher power which enables transmission links to be maintained in poor transmission conditions or over longer distances.
The system preferably additionally comprising means for generating a second modulated signal by applying an amplitude dependent modulation scheme to data to be transmitted across higher quality transmission links and amplifier means for linearly amplifying the second modulated signal. Thus, for higher quality links a higher spectral efficiency can be achieved by the use of amplitude dependent modulation schemes which have to be amplified substantially linearly if the data on them is to be successfully recovered.
According to a second aspect of the present invention there is provided a cellular radio communication system for transmitting data over a plurality of transmission links comprising:
means for generating a first modulated signal by applying an amplitude dependent modulation scheme to data to be transmitted across poor quality transmission links;
means for generating a second modulated signal by applying an amplitude dependent modulation scheme to data to be transmitted across higher quality transmission links; and
amplifier means for non-linearly amplifying the first modulated signal and for linearly amplifying the second modulated signal.
The second aspect of the present invention has the same advantages as the first aspect in that high quality transmission links are allocated a high spectral efficiency amplitude dependent modulation scheme and low quality transmission links are allocated an amplitude dependent modulation scheme so that the power of signals across the low quality transmission links can be increased by non-linear amplification of the signals transmitted across it.
The constant amplitude envelope modulation scheme may be GMSK or where conditions allow the more spectrally efficient Multiple Level Continuous Amplitude Modulation (MLCAM).
The means for generating a second modulated signal preferably applies a Quadrature Phase Shift Keying (QPSK) modulation scheme, a 16-QAM (Quadrature Amplitude Modulation) modulation scheme or a 64-QAM (Quadrature Amplitude Modulation) modulation scheme to data to be transmitted across higher quality transmission links depending on the quality of the links and amplifier means for linearly amplifying the second modulated signal. In this way the bandwidth of the system is used efficiently by adaptively selecting the most spectrally efficient modulation scheme for a link which can provide a desired bit error rate.
Preferably the system comprises means for measuring the quality of the transmission links and selecting modulation schemes depending on the measured quality. In this way as the quality of a transmission link varies with time the system is responsive to allocate the most appropriate modulation scheme.
In a preferred embodiment of the system each transmission link is allocated a default modulation scheme for use when a call is initiated which default modulation scheme is the highest spectral efficiency modulation scheme which will deliver a predetermined bit error rate in poor environmental conditions. Thus, calls can be initiated using a default modulation scheme which will deliver the desired bit error rate even in poor environmental conditions. A different modulation scheme may be selected for the call subsequently in response to the measured quality of the transmission link.
The means for generating a second modulated signal may apply a level of forward error correction coding to the data to be transmitted across higher quality transmission links as this will improve the efficiency with which the bandwidth of the system is used.
The transmission links may comprise a point to multi-point link and the system may be a wireless access system.
According to a third aspect of the present invention there is provided a transmitting unit for a cellular radio communication system for transmitting data over at least one transmission link comprising:
means for generating a first modulated signal by applying a constant amplitude envelope modulation scheme to data to be transmitted across a poor quality transmission link;
means for generating a second modulated signal by applying an amplitude dependent modulation scheme to data to be transmitted across a higher quality transmission link; and
amplifier means for non-linearly amplifying the first modulated signal and for linearly amplifying the second modulated signal.
The transmitting unit has the same advantages and the same preferred features as the system according to the present invention. In particular the transmitting unit may comprise means responsive to communications from a receiving unit about the quality of a transmission link wherein said means selects a modulation scheme for the transmission link depending on the communication.
According to a fourth aspect of the present invention there is provided a receiving unit for a cellular radio communication system for receiving data over at least one transmission link comprising:
means for recovering data from signals modulated according to a constant amplitude envelope modulation scheme; and
means for recovering data from signal modulated signal modulated according to an amplitude dependent modulation scheme.
The receiving unit has the same advantages and the same preferred features as the system according to the present invention. The receiving unit preferably comprises means for measuring the quality of a transmission link from a transmitting unit and means for communicating the measured quality to a transmitting unit.
According to a fifth aspect of the present invention there is provided a method of operating a cellular radio communication system for transmitting data over a plurality of transmission links comprising the steps of:
generating a modulated signal by applying a constant amplitude envelope modulation scheme to data to be transmitted across poor quality transmission links; and
non-linearly amplifying the modulated signal.
The method may additionally comprise the steps of:
generating a second modulated signal by applying an amplitude dependent modulation scheme to data to be transmitted across higher quality transmission links; and
linearly amplifying the second modulated signal.
The method has the same advantages as the system according to the present invention.